The improvements of our invention may be applied to a hydrokinetic torque converter transmission of the kind disclosed, for example, in U.S. Pat. Nos. 4,978,328, 4,637,281, and 5,029,087, which are assigned to the assignee of this invention. Those references disclose a four-speed transaxle transmission for use in an automotive vehicle driveline. The transaxle has two simple planetary gear units arranged on a first axis transversely disposed with respect to the center plane of the vehicle and a torque transfer drive between torque output elements of multiple ratio gear units to each of two traction wheels through half shaft assemblies in a front wheel drive arrangement. The engine is mounted with its crankshaft arranged in spaced parallel relationship with respect to the axis of the planetary gearing. A hydrokinetic torque converter, mounted on the crankshaft axis, is connected by means of a chain drive to torque input elements mounted on the planetary gearing axis.
The torque converter of these prior art designs has a controllable bypass friction clutch that is engageable to establish a mechanical torque transfer between the crankshaft of the engine and the turbine of the converter, thus bypassing the hydrokinetic torque flow path.
The improvements of this invention can be applied also to a rear wheel drive transmission of the kind shown for example in U.S. Pat. No. 4,934,216. That transmission includes a hydrokinetic torque converter and multiple ratio gearing mounted on an axis that is common to the engine crankshaft axis. The torque converter of the design shown in the '216 patent also includes a friction bypass clutch.
It is desirable to control the capacity of the bypass clutch to effect a controlled slip in the clutch to compensate for torque transients and to eliminate noise vibration and harshness in the driveline. U.S. Pat. No. 5,029,087 describes one method for achieving a controlled slip in a bypass clutch. The clutch of that reference establishes a controlled slip condition by relying on engine speed and turbine speed signals. It continuously monitors the difference between the actual converter slip at any instant and a desired slip. That error is used to compute a duty cycle for a solenoid-operated pressure controller for the bypass clutch. The slip is controlled in this fashion until a final target slip is achieved. The target slip is a value stored in a microprocessor memory. Its magnitude depends upon throttle position and vehicle speed.
A microprocessor controller is used in a closed loop control circuit to establish partial clutch engagement rather than a full mechanical torque bypass through the clutch during a major portion of the operating time. The bypass clutch will permit the converter to operate near 100% mechanical efficiency when the driveline is operating in a steady-state mode.
The microprocessor responds to converter slip which is computed by sensing the turbine shaft speed and the engine speed and determining an error in the slip, the error being the difference between the actual slip and a desired slip. The desired slip depends upon information from a throttle position sensor, an engine speed sensor, a gear shift selector sensor, an oil temperature sensor, a vehicle speed sensor, and a transmission input shaft speed sensor.
In related copending patent application Ser. No. 922,627, filed Aug. 10, 1992, a modulated bypass clutch controller is described. That controller will achieve a controlled slip condition after the bypass clutch is commanded by the transmission control system to engage. After a steady-state condition is achieved, the controller will effect a so-called desired slip that will equal the target value. After the target value is reached and steady-state conditions continue, the desired slip may be reduced to zero or near zero to eliminator substantially eliminate slip in the hydrokinetic unit, thereby providing an added improvement in overall transmission operating efficiency. This also tends to improve durability of the clutch by providing more efficient torque energy management as friction heat is reduced.
Application Ser. No. 926,627, Aug. 10, 1992, was filed by A. L. Leonard, Kenneth, Walega and David Garrett, John Daubenmier, Bruce Palansky, Thomas Greene and Lawrence Buch, and is entitled "Automatic Transmission Control System". It is assigned to the assignee of the present invention. That application discloses a bypass clutch arrangement in a multiple ratio transmission wherein a close loop control of the bypass clutch capacity is effected by establishing a desired slip during steady-state operation following a command for clutch engagement. The desired slip is determined by setting it equal to the actual measured slip and then ramping the desired slip to achieve a progressively decreasing converter slip until a target value is reached. After steady-state operation is achieved and the desired slip is equal to target slip, the controller will cause a transition from the open loop slip control into a so-called "hard lock" mode in which the desired slip is again ramped to provide progressively decreasing actual slip until a zero slip or a near zero slip is effected. The near-zero slip condition is sometimes referred to as a "soft lock" mode.
The transmission disclosed in the copending application identified above does not include a turbine speed sensor, but it does include a vehicle speed sensor. It thus is necessary for purposes of carrying out the bypass clutch control strategy to compute, during each background control loop of the microprocessor, a turbine speed using a current vehicle speed value and a current gear ratio value for the transmission gearing.
The control strategy for the bypass clutch control disclosed in the copending application identified above makes provision for modulating the bypass clutch pressure during shift intervals. This is done to avoid undesirable torque transients during the shift. It is necessary in the design of the copending application to continuously monitor calculated speed ratio across the converter. When a predetermined delta speed ratio, or speed ratio difference, is detected, that is an indication of the beginning of a shift following the command of a shift by the control system processor. Pressure modulation occurs, following the detection of the beginning of the shift, through the shift interval and is ended when the processor detects that the so-called delta speed ratio is sufficiently high to indicate that the shift actually has come close to completion. The processor then will return to the close loop control, and that in turn is followed by the so-called hard lock mode or soft lock mode described above.
The clutch control of the present invention has features that are common to the controller and the control strategy described in the copending application. Those common features relate in general to the behavior of the bypass clutch following a command for clutch engagement and prior to a command for a change in speed ratio. Those common features relate also to the behavior of the clutch after the completion of a shift interval. The transmission of the present invention includes a turbine speed sensor, and in this respect it differs from the transmission described in the copending application. Thus, the behavior of the clutch and the control strategy for effecting control of the clutch during the shift interval differs from that described in the copending application.